The present invention relates to radiofrequency (RF) ablation of tumors and, in particular, to an apparatus and method for safely ablating large tumors, for example, within organs such as the liver.
Malignant liver tumors are a substantial problem in the United States and worldwide. Virtually all patients who succumb to colorectal cancer have evidence of metastatic tumors in the liver at the time of death resulting from the filtering and scavenging mechanism of the liver.
Large studies demonstrate a five-year survival advantage in 33% of the cases in which all macroscopic tumors of liver-only metastatic colorectal carcinomas are removed. In contrast, treatment by chemotherapy alone results in approximately zero percent, five-year survival rates.
Nevertheless, surgical resection or removal of the tumors is not always possible. Liver lesions, located deep in the hepatic parenchyma are usually not amenable to wedge resection and thus require a segmentectomy or lobectomy for complete removal. Resection of multiple, deep lesions may require the loss of an unacceptable amount of hepatic parenchyma. Extensive hepatic resections increase post-operative morbidity and mortality, including post-operative hepatic failure. In addition, the need for transfusion during hepatic resection increases the tumor recurrence rate, probably due to poorly understood immunological mechanisms.
In cases where surgery is not practical, RF ablation for tumor destruction may be preferred, yet it can be difficult to ablate large tumors reliably, especially in perivascular tissue and at tumor margins. Although ablation induced temperatures above 60° centigrade are almost instantaneously cytotoxic, it takes four to six minutes for cells to undergo irreversible change when exposed to 50° centigrade. It is difficult to provide uniformly high temperatures throughout a large tumor volume, particularly near blood vessels, which provide a heat sink, and near the edges of the tumor, leading to a substantial risk of incomplete treatment of the tumor. Given the highly vascular nature of the liver, most large tumors will be in close proximity to at least one major blood vessel.